In recent years, the mechanism by which mammalian immune systems, such as human and murine systems react to infections, foreign antigens, and to so-called "self antigens" in connection with autoimmune diseases has begun to be established. See, in this regard, Grey, et al., Scientific American 261(5): 56-64 (1989); Male, et al., Advanced Immunology (J. P. Lippincott Company, 1987), especially chapters 6 through 10.
Well known, both to the skilled artisan and to the general public is the role of antibodies, sometimes referred to as "immunoglobulin" or the less correct and older "gammaglobulin" in response to infection. Antibodies are protein molecules which are produced by B cells in response to infection. It is well known that these antibodies act to "disable" or to inactivate infectious agents in the course of combating the infection.
In order for antibodies to be produced, however, preceding events must occur which lead to stimulation of the B cells which produce the antibodies. One of the key events involved in the processes leading to antibody production is that of antigen recognition. This aspect of the immune response requires the participation of so-called "T-cells" and is less well known than the antibody response commented on supra.
Briefly, and in outline form, antigen recognition requires interaction of an "antigen presentation cell", a "processed antigen", and a T-cell. See Grey and Male, supra. The "processed antigen" in an infection, is a molecule characteristic of the pathogen which has been treated, i.e., "processed", by other cells which are a part of the immune system. The processed antigen interacts with a receptor on the surface of a presenting cell in a manner not unlike a lock fitting into a key hole or, perhaps more aptly, two pieces of a jigsaw puzzle.
The configuration of the complex of processed antigen and receptor on an antigen presenting cell allows the participation of T-cells. T-cells do not join the complex unless and until the processed antigen has fit into the receptor on the antigen presenting cell. This receptor will hereafter be referred to by its scientific name, the major histocompatibility complex (MHC), or the human leukocyte antigen (HLA). Generally, MHC is used to refer to murine systems, and HLA to humans.
These receptors fall into two classes. MHC-II molecules are involved in most responses to pathogens. In contrast, MHC-I molecules are involved when the pathogen is a virus, or a malignant cell is involved. When MHC-I participation is involved, there is no antibody stimulation; rather, the interaction of MHC-I, processed antigen and T-cell leads to lysis of cells infected with the pathogen.
The foregoing discussion has focused on the events involved in responding to "infection", i.e., the presence of pathogenic foreign material in the organism. Similar mechanisms are involved in autoimmune diseases as well. In these conditions, the organism treats its own molecules as foreign, or as "self-antigens". The same type of complexing occurs as described supra, with an antibody response being mounted against the organism itself. Among the diseases in which this is a factor are rheumatoid arthritis, diabetes, systemic lupus erythromatosus, and others.
The ability of the T-cell to complex with the processed antigen and MHC/HLA complex is dependent on what is referred to as the T-cell antigen receptor, referred to as "TCR" hereafter. The TCR is recognized as a heterodimer, made up of alpha (.alpha.) and beta (.beta.) chains. Five variable elements, coded for by germline DNA and known as "V.alpha., J.alpha., V.beta., D.beta. and J.beta." as well as non-germline encoded amino acids contribute to the TCR. See, in this regard, Marrack et al., Immunol. Today 9: 308-315 (1988); Toyonaga et al., Ann. Rev. Immunol. 5: 585-620 (1987); Davis, Ann. Rev. Immunol. 4: 529-591 (1985); Hendrick et al., Cell 30:141-152 (1982). With respect to the binding of TCR with processed antigen and MHC, see Babbitt et al., Nature 317:359-361 (1985); Buus et al., Science 235: 1353-1358 (1987); Townsend et al., Cell 44: 959-968 (1986); Bjorkman et al., Nature 329: 506-512 (1987).
Generally, both the alpha and beta subunits are involved in recognition of the ligand formed by processed antigen and MHC/HLA molecule. This is not always the case, however, and it has been found that so-called "superantigens" stimulate T-cells with a particular V.beta. element, regardless of any other element. See Kappler et al., Cell 49: 273-280 (1987); Kappler et al., Cell 49: 263-271 (1987); MacDonald et al., Nature 332: 40-45 (1988); Pullen et al., Nature 335: 795-801 (1988); Kappler et al., Nature 332: 35-40 (1988); Abe et al., J. Immunol. 140: 4132-4138 (1988); White et al., Cell 56: 27-35 (1989); Janeway et al., Immunol. Rev. 107: 61-88 (1989); Berkoff et al., J. Immunol. 139: 3189-3194 (1988), and Kappler et al., Science 244: 811-813 (1989).
The "superantigens" mentioned supra, while generally stimulating T-cells as long as they possess a V.beta. element, are somewhat specific in terms of the particular form of the V.beta. moiety which is present on the stimulated T cell.
The V.beta. element may be any of a number of different members of a related family of such elements. Different members of the family have different effects. See, in this regard Choi et al., Proc. Natl. Acad. Sci. USA 86: 8941-8945 (1989), which showed that T-cells bearing human V.beta. 13.2 respond to antigen SEC 2, while closely related V.beta. 13.1 does not induce that type of result.
Study of the different members of the V.beta. family requires the investigator to be able to differentiate between them. Many of the individual family members, such as V.beta. 13.1 and V.beta. 13.2 differ by only a few amino acids, however, making differentiation extremely difficult.
Antibodies, monoclonal antibodies in particular, are known for their ability to bind to specific protein molecules, allowing differentiation between even very closely related proteins. Differentiation of alternate or variant forms of oncogene proteins, glycoprotein hormones, and even V.beta. elements using monoclonal antibodies is known to the art. In the case of V.beta. elements, however, very few hybridomas are known which produce the desired mAbs, and those which are known are generally unstable. Since it is known that there are well over 30 different forms of V.beta. elements already known (see Kappler et al., supra), and one can expect this number to increase, it is important to have a method available which will generate the desired monoclonal antibodies.
The standard approach to the generation of monoclonal antibodies is essentially that first described by K ohler and Milstein, Nature 256: 495-497 (1975). Essentially, this methodology involves immunizing a subject animal, generally a mouse, rat or rodent with the material against which the antibodies are desired. The recipient's immune system generates antibodies against the material (the "immunogen") via its B cells. These are localized in the animal's spleen. The spleen is removed and treated to separate it into individual cells. Following this, the cells are fused with cell lines which are immortal in culture, generally myeloma cells, in the presence of an agent which facilitates fusion, generally polyethylene glycol. Successful fusions yield hybridomas. Not all of the hybridomas will produce the desired antibody, and it then becomes necessary to screen the hybridomas by assaying them, generally with the antigen in question, to determine their specificity. Since the hybridomas produce only one form of antibody, these are referred to as monoclonal antibodies being, the antibodies derived from a single clone.
Critical for the generation of an antibody response is the recognition by the host animal of the immunogen as foreign. A host's immune system will not respond to a molecule recognized as being identical or very similar to one of its own molecules, under normal conditions. It is well recognized that the human and mouse immune systems are very similar, and generation of human specific antibodies is therefore more difficult than usual when immune receptor molecules are involved.
In the case of T-cell receptors, an additional difficulty exists in that the various members of this family differ to a very small degree, sometimes by less than 20 amino acids. While this degree of difference is key to the mounting of a proper immune response, the small degree of difference may not be sufficient to generate a monoclonal antibody which binds specifically to one member of the family, to the substantial exclusion of others.
Various approaches have been applied in attempting to generate monoclonal antibodies against "difficult" antigens, such as those described supra. Many of these approaches are summarized in Goding, Monoclonal Antibodies: Principles and Practice (Academic Press; 1988), the disclosure of which is incorporated by reference, especially pages 1-93. Among the methodologies used are the coupling of the immunogen to a foreign molecule, such as bovine serum albumin, or the use of various adjuvants and other additives which seem to improve the immune response. Such approaches, however, are frequently "hit or miss" and there is little success guaranteed. The art still looks for a systematic approach to this issue.
The early work in monoclonal antibody technology featured the use of whole cells as immunogens. As the field developed, and protocols for purifying individual molecules of interest became more established, protocols in immunology stressed the use of the molecule, rather than the cell. This is because a whole cell features an uncountable array of antigenic sites for generation of antibodies. The investigator's chance of securing an antibody to a particular molecule is remote.